For this achievement select a character with the powerhouse skill. A few people that have it are the starting character Marcus, lily's brother that you rescue and Martin Cerny.

Once you select them train their powerhouse skill to level 6. Then Select the double kill skill in the defence skill section. (This is at the bottom of your skill page) hold X to select the double kill skill ( make sure you select the double kill skill and not the other one because then you cant get the double kill skill.

Then find two zombies wait for them to get close and hit LB and B on the controller and if they are close enough you should preform the double kill skill on the zombies.

Here's a vid